Unexplored Pathways To Charge Storage in Supercapacitors

Abstract

This study reveals various unexplored pathways to energy storage in parallel and curved plate supercapacitors (SCs). The spatiotemporal variations in the electric field intensity of such SCs were found to have a significant influence on their performance. The observations unearth the limitations associated with the previous theoretical models, which are routinely employed to analyze the performance of SCs by considering electrical double layers (EDLs) as capacitors near the electrodes. The time-dependent electrochemical behaviors of SCs obtained from the Nyquist and Bode diagrams of electrochemical impedance spectroscopy showed (i) electrode polarization at the higher-frequency sweeps, (ii) immobile Helmholtz layer formation at the mid-frequency zone, and (iii) formation of diffuse layer of EDL in the low-frequency regime. The results suggest that charge storage of SCs heavily depend on electrode geometry, type of electrolyte, electrolyte concentration, electrode separation, separator type, and dielectric relaxation of the electrolyte. A theoretical model composed of Poisson–Nernst–Planck equations for the electric field in electrolyte and Laplace equation for the electric field in electrodes coupled with Navier–Stokes equations for the fluid flow was numerically solved with appropriate boundary conditions to uncover the pathways to supercapacitance during the experiments. The experimental and theoretical studies together reveal that the use of the potential drop across the EDL originating from the opposing electric fields due to electrode polarization and EDL formation could provide more accurate pathways to supercapacitance of such SCs.